1. Field of the Invention
The present invention relates to an alignment mark, a method of manufacturing a semiconductor device, and a mask set.
2. Description of the Related Art
A stacked cross-point memory such as a resistance random access memory (ReRAM) is configured by stacking memory cells having the same pattern a plurality of times. When such a stacked cross-point memory is manufactured, alignment marks are formed in respective layers in advance. When a pattern in an upper layer side is formed, mask alignment (alignment of the upper layer with the lower layer) is performed by using an alignment mark formed in a pattern on a lower layer side.
When it is attempted to stack memory cells of the stacked cross-point memory using only one mask set, the same mask is used a plurality of times. Therefore, a memory cell pattern on the upper layer side is stacked on a memory cell pattern on the lower layer side. The alignment mark on the upper layer side is also stacked on the alignment mark on the lower layer side. Therefore, after memory cells of two or more layers are formed, when a word line and a bit line are aligned with a memory cell of an uppermost layer, an alignment mark formed in a memory cell is present in a lower layer side of an alignment mark of the uppermost layer. When position detection signals for detecting the positions of the alignment marks is irradiated, the position detection signals interfere with each other in the alignment mark on the upper layer side and the alignment mark on the lower layer side. This makes it difficult to accurately detect the positions of the alignment marks. In this way, in the stacked cross-point memory, although the memory cells in the respective layers are the same pattern, the memory cells cannot be stacked with only one mask set.
Therefore, even if internal data of memory cell patterns are the same, to align memory cells, it is necessary to prepare mask sets of memory cells equivalent to the number of stacks having different arrangements of alignment marks. Respective masks of the memory cells equivalent to the number of stacks are masks having the same data of the memory cells and different arrangements of alignment marks (see, for example, Japanese Patent Application Laid-Open No. H2-158120).
However, in the method disclosed in Japanese Patent Application Laid-Open No. H2-158120, it is necessary to manufacture mask sets of memory cells equivalent to the number of stacks. Therefore, a large number of masks are used to form the memory cells and high cost is required to manufacture and maintain the mask sets.